Thin films of organic semiconductor compounds are applied to functional materials for, for example, electro-optical devices, semiconductor devices, light-emitting devices, and laser media. Particularly in recent years, investigation of new functional materials expecting the diversity held by the organic semiconductor compounds is being discussed. For the purpose of favorably using the organic semiconductor compound as the functional material, it is preferable that the organic semiconductor compound is formed in the state of a thin film on a substrate onto which electrodes and the like are provided.
On the other hand, the organic semiconductor compound is used in a crystalline state or in an amorphous state. In general, the organic semiconductor compounds in the crystalline state have tendency of having high semiconductor characteristics such as mobility, but it is difficult to prepare the crystal with a large area and without any defect. Resultantly, under the current situation, there are many cases where the organic semiconductor compounds in the amorphous states are used.
Thus, necessity for preparing the organic semiconductor compound in the form of a highly crystalline thin film have raised.
Hitherto, as techniques for preparing thin films, some methods are proposed, including, for example, vacuum vapor deposition methods represented by a molecular beam epitaxy (MBE) method or a pulsed laser vapor deposition (PLD) method; a spin coat method, in which a solution prepared by adjusting the composition ratio of elements to be added is applied onto a substrate, followed by heating and drying; and an ink-jet method, in which such a solution is applied by ink-jet. Further, there are also proposed a technique of allowing a melt to change into a thin film by cooling slowly, and a technique of allowing the solution to change into a thin film by slowly evaporating a solvent therein or by cooling slowly.
However, since the highly crystalline thin film is obtained from the solution via crystal growth, it is difficult to prepare the highly crystalline thin film by merely applying the conventional vacuum vapor deposition method as it is. Further, it is difficult to control the film thickness with nanometer unit via the conventional spin coat method and ink-jet method, and the resultant film is non-uniform in thickness and moreover it is poor in crystallinity of the resultant thin film. Furthermore, it is difficult to control the film thickness via the conventional method for crystal growth from the melt or solution.
In recent years, a method is proposed, in which a precursor material applied on a substrate placed in a vacuum condition is sprayed with a solvent vapor, to evaporate and crystallize the precursor (for example, see Journal of Applied Physics, 2001, Vol. 90, No. 1, p. 209-212.). According to this method, since the solvent vapor is sprayed in vacuo, there is a merit that the concentration of the solvent vapor can be measured via its pressure.
Further, a method is proposed, in which a precursor material of a thin film to be prepared is adhered on a substrate, the solvent used for dissolving the precursor material is atomized to generate a solvent vapor, and the thus-generated solvent vapor is brought to contact with the precursor material, thereby to crystallize the precursor under the atmospheric pressure (for example, see JP-A-2005-281011 (“JP-A” means unexamined published Japanese patent application)). In this situation, crystallization of the precursor material is controlled by adjusting the concentration of the solvent vapor to generate. According to the method, the crystallization process can be carried out under the atmospheric pressure, and it is also possible to conduct patterning in a nano-sized order via the ink-jet technology.
Furthermore, a method is proposed, in which an organic solvent, in which an organic semiconductor compound is soluble, is applied on a substrate, to form a liquid film, the organic semiconductor compound is brought to adhere thereto by, for example, vapor deposition or sputtering, to have the compound take through the state that the compound is dissolved in the organic solvent, and then the organic solvent is evaporated, thereby to prepare a thin film of the organic semiconductor compound (for example, see Crystal Growth Des. 2003, Vol. 3, p. 125, and Physical Review letters, 1998, Vol. 81, No. 2, p. 622.).
However, according to those conventional methods, it is still difficult to produce the single-crystal thin film of the organic semiconductor compound in a large area and uniformly. For example, Physical Review letters, 1998, Vol. 81, No. 2, p. 622 reports that tetracene crystallizes as the organic semiconductor material in toluene, but the crystallized area of the thin film is small and the crystals do not distribute uniformly, and it is not necessarily in the satisfactory level.
Further, in the above method, in which an organic solvent, in which an organic semiconductor compound is soluble, is applied on a substrate, to form a liquid film, and then the organic semiconductor compound is brought to adhere thereto by, for example, vapor deposition or sputtering, there is a case where the surface of the organic solvent liquid film thus formed on the substrate waves, that is, so-called oscillatory phenomenon occurs, upon adhering the organic semiconductor compound on the substrate. When the oscillatory phenomenon occurs, defects tend to occur and the crystal growth direction tends to become random, upon the formation of crystals of the organic semiconductor, which results it impossible to stably produce the single-crystal thin film in a large area and uniformly (for example, see Langmuir, 2007, vol. 23, p. 6864, etc.). Langmuir, 2007, vol. 23, p. 6864 reports that it is presumed such an oscillatory phenomenon is involved in a contact angle between the substrate surface and the organic solvent, and that the oscillatory phenomenon can be suppressed when the contact angle is 30 degrees or smaller.